Huxley: Glands and Development 



THYROID FED TADPOLES AND CONTROL 



Figure 2. Larvae of the bull frog normally retain the tadpole form for two or even 

 three years before metamorphosis takes place. The changes produced in the three "half-and- 

 half" individuals shown were the result of twenty-six days of thyroid feeding. Notice the 

 great decrease in size that has occurred, the tissues being literally broken down and burned 

 up under the more rapid metabolism produced by increased concentration of thyroid secre- 

 tion. Photograph reproduced by permission of Dr. W. W. Swingle, and the Journal of 

 Expcriiiiciital Zoology. 



thyroxin. Allen'' seems inclined to 

 adopt the same view. 



This hypothesis is based chiefly on 

 the extremely important fact discovered 

 by Swingle'" and already referred to, 

 that inorganic iodine produces meta- 

 morphosis in Anuran tadpoles, and 

 this whether they are normal or have 

 had their thyroids removed. 



However, even in adult mammals it 

 has been shown that inorganic iodine 

 in the blood-stream is very rapidly 

 taken up by the thyroid, and is quickly 

 built into some specific organic com- 

 pound (presumably thyroxin). In 

 mammals, therefore, iodine is promptly 

 elaborated by the organism to form 

 part of a complex substance which is 

 the thyroid hormone. 



So far as one can judge, exactly the 



same process is at work in larval Anura. 

 The provision of an amount of iodine 

 greater than the normal, up to the high- 

 est concentration tolerated by the ani- 

 mals, results in an increase in the size 

 of the thyroid, and in the amount of 

 its colloid content.'" On the other 

 hand, when one large dose of thyroid 

 substance is given as food, metamor- 

 phosis occurs, but the thyroid remains 

 small, increasing very little, if at all, 

 in volume.* In other words, an excess 

 of thyroid hormone acts directly and 

 does not require to be worked up by 

 the animal's own thyroid gland, where- 

 as an excess of free iodine, however 

 great, is worked up by the animal's 

 own glands, presumably into the nor- 

 mal thyroid hormone, before exerting 

 its metamorphic effects. 



